1. Field of the Invention
This disclosure relates generally to devices for estimating blood alcohol content from a breath sample, and more particularly, to housings for assemblies that utilize a fuel cell for estimating blood alcohol content from a breath sample.
2. Description of the Related Art
An alcoholic beverage is a drink containing ethanol, commonly known as alcohol, although in chemistry the definition of alcohol includes many other compounds. Alcohol, specifically ethanol, is a psychoactive drug and is a powerful central nervous system depressant with a range of side effects.
Alcohol has a biphasic effect on the body, which is to say that its effects change over time. In the initial stages of intoxication, alcohol generally produces feelings of relaxation and cheerfulness. Further consumption however affects the brain leading to slurred speech, blurred vision, clumsiness and delayed reflexes, among other coordination problems. This condition is commonly referred to as intoxication or drunkenness, and eventually subsides when the alcohol has fully metabolized in the body.
When a human drinks alcohol, the alcohol housed in the stomach passes into the bloodstream. Cell membranes are highly permeable to alcohol, so once alcohol is in the bloodstream it can diffuse into nearly every biological tissue of the body. Once in the bloodstream, the alcohol circulates to the brain, resulting in intoxication, loss of inhibition and impairment of motor skills such as driving a vehicle. The amount of alcohol consumed and the circumstances surrounding consumption play a large role in determining the extent of an individual's intoxication. Examples of such circumstances include the amount of food in the stomach at the time of alcohol consumption and the hydration level of the individual at the time of consumption, among others.
Due to the coordination impairment and other symptoms associated with intoxication and drunkenness, most countries have laws against drunk driving, i.e., driving with a certain concentration of ethanol in the blood. The legal threshold of blood alcohol content ranges from 0.0% to 0.08%, depending on the jurisdiction. Punishments for operating a vehicle over the legal limit in a given jurisdiction generally include fines, temporary loss of an individual's driving license and imprisonment. Creation of these laws has led to a market for devices to accurately measure the blood alcohol content of individuals operating motor vehicles.
Blood alcohol content (BAC) or blood alcohol concentration is the concentration of alcohol in the blood (weight per unit volume). While blood alcohol content can be directly measured in a hospital laboratory setting, it is more common for it to be measured in law enforcement situations by estimation from an individual's breath alcohol concentration using a breath alcohol testing machine.
In the world of alcohol-breath testing and related fields of alcohol testing, one of the most common configurations uses fuel cells as alcohol sensors, with the assembly allowing breath, air, gas, or vapor to pass through or into the fuel cell. The fuel cell is generally encased/enclosed in some type of housing which includes two sensor ports—an intake port and a port for a mechanical means for moving the sample through the fuel cell sensor. The mechanical means is generally separate from the fuel cell sensor itself; but such separation is not necessary. In some cases, the mechanical means might only move breath or air in one direction through the fuel cell, as shown in FIG. 1; in other cases, the mechanical means can move the breath/air back and forth in two directions, as shown in FIG. 2.
The mechanical means can take on a variety of forms, including, but not limited to, a diaphragm, a bellow, a piston, or a pump. For example, the mechanical means may be a piston that is able to move a small, fixed volume sample of high accuracy through the sensor in a very short amount of time (e.g., a fraction of a second with the single stroke of a piston) or the piston may use multiple “strokes” to move a larger, less accurate amount of sample over a longer period of time (e.g., a continuous pump operating over several seconds). Moreover, the mechanical means may be connected to the fuel cell by any variety of tubes, connections, or the like and may incorporate any variety of valves, check valves, or directional valves. In any event, the description of the mechanical means here is merely exemplary and not meant to be all-inclusive.
The sensor ports discussed above are generally kept relatively small when compared to the volume of the fuel cell sensor itself. Such a configuration helps isolate the sensor volume from other interior volumes within the assembly and reduces the overall dead space in the assembly.
In its simplest form, the alcohol fuel cell sensor consists of a chemically inert, porous material coated on both sides with thin layers of catalyst such as platinum (forming upper and lower platinum electrode layers). The porous material sandwiched between the two platinum layers contains a liquid acid-electrolyte. Those skilled in the art, however, would understand that the chemically inert porous material, filled with liquid acid-electrolyte, could be replaced by, under certain circumstances, a solid electrolyte element. In any event, the electrolyte allows charges to move between the two electrode layers. Round, platinum wire electrical connections are then applied to the platinum electrodes and connected to an external circuit. As noted above, the entire fuel cell sensor is mounted in a plastic case, which is provided with a gas inlet port that allows a breath sample to be introduced into the assembly. The basic configuration is as described above, and illustrated in FIG. 3.
In the fuel cell sensor, the top platinum electrode (the one closest to the gas inlet) oxidizes any alcohol in the breath sample to produce acetic acid in a 2 step process (ethanol→acetaldehyde→acetic acid) and which also produces free electrons in the process. Hydrogen ions (H+) are also freed in the process, and migrate to the lower platinum electrode of the cell, where they combine with atmospheric oxygen to form water, resulting in a deficiency of electrons on the lower electrode equal to the excess of electrons produced on the upper surface. Because the two electrode surfaces are connected electrically, a current flows through this external circuit to neutralize the charge. With suitable amplification, this current is a precise indicator of the amount of alcohol consumed by the fuel cell, as the number of electrons produced is directly and linearly proportional to the number of alcohol molecules arriving at the catalyst surface. With the number of alcohol molecules, the blood alcohol content can then be determined. This process is illustrated in FIG. 4.
When passing a breath sample into, and especially when passing a sample through the sensor, it is advantageous that the breath sample be spread across as much of the surface of the electrode as possible. This allows the entire electrode surface area to react with the breath sample which results in a more accurate reading. Additionally, when the sample is moved through the sensor, it is important that all the alcohol in the breath sample be attached to the electrode surface for reaction and that none of the key portion of the breath sample (i.e., the alcohol) passes through the sensor without participating in the reaction. This allows for a quicker and more accurate reading on whatever device incorporates the fuel cell sensor for measurement purposes.
There have been attempts in the prior art to ensure the contact between the breath sample and electrode surface area, but they all have their own problems. As shown in FIG. 5, one option is to have the sample intake port offset from the port for the mechanical means. Another option, as shown in FIG. 6, is to use baffles. Generally speaking, these designs do a reasonable job of spreading a fully gaseous, non-condensing sample across the surface of the electrode as discussed above. However, these designs begin to show weakness when the sample contains any liquid.
Breath samples are often contaminated with liquid (e.g., saliva in breath) or create condensation in a sensor due to the extremely high relative humidity of a breath sample (e.g., 34° C. breath sample passing through a sensor with a lower ambient temperature, e.g., 25° C.). Heating the sensor can solve condensation issues at the cost of a more expensive, more complicated design with high energy use when compared to a sensor without heating. Heating the sensor is a less effective strategy when the sample contains actual liquid in the form of large drops which often occurs from those who are intoxicated. Sometimes heating the sensor is not even enough to avoid condensation issues. If condensation occurs in other portions of the breath path that are not heated, liquid might still enter the heated sensor from a prior condensate being forced by the moving gas of the breath into the sensor.
A thin layer (e.g. 5 mil) of gas-permeable membrane or material that is waterproof, thin, stretchable, and compatible with acids (e.g., GORE-TEX®) has been utilized in the prior art to separate the active platinum electrode surface from the remainder of the internal volume of the sensor, as shown in FIG. 7. Generally, the GORE-TEX® layer is placed close to (but not in contact with) the platinum electrode surface to minimize any dead space between the GORE-TEX® and the sensor surface. The GORE-TEX® layer thus serves as a “regulator” to control the rate at which the breath sample reaches the electrode, and to slow down the reaction. It also acts as a moisture barrier such that liquid cannot pass through the membrane to the electrode sensor.
The moisture barrier ensures both that the sensor does not become contaminated with liquid (e.g., saliva in breath) and condensation and that liquid does not enter the electrolyte within the sensor. In these assemblies, the moisture barrier of the GORE-TEX® layer collects the residual liquid and condensation on the surface thereof with the liquid getting “trapped” in the assembly without contacting the electrode. However, a continuously wet area of the GORE-TEX® layer prevents the breath sample from passing efficiently through the GORE-TEX® membrane, can absorb alcohol from, or exude alcohol to, a subsequent gas sample, and ultimately will affect subsequent sensor readings. Moreover, having a GORE-TEX® layer that is sometimes wet and sometimes dry (e.g. depending on how much the breath sensor has been used recently) further affects the sensor accuracy. Therefore, there is a need for an assembly that spreads the sample across the electrode in a predictable fashion and lends itself to easy flushing of this residual liquid and with an efficient and air-tight seal for the GORE-TEX® layer to ensure accurate alcohol readings.